1. Field of the Invention
The present invention relates to the generation of electrical power from submerged generators using tidal currents to turn the generators, but it may be adapted to more powerful ocean current applications.
2. Description of the Prior Art
The decrease in the amount and production of world crude oil, and North America natural gas, presents a potential catastrophic problem for the industrial world. While the amount and location of crude oil reserves are not well documented, it is acknowledged that there is limited amount of the fossil fuels in reserve. In addition, many feel that the combustion of the fossil fuels for transportation and energy production contributes to global warming. It would therefore be a worthwhile thing to switch from fossil fuels for energy production to energy sources which are self-sustaining and non-polluting. Other recently investigated possibilities for renewable resources include wind, solar, geothermal, and biomass. A couple of these show real potential, and are either in use or under further development. Ocean wave and tidal currents are probably the last of the large natural resources not yet fully investigated for the production of energy, and in particular, electricity. This disclosure relates to using tidal currents to generate electricity.
The United States, with coast lines on both sides and a network of inland rivers and lakes, has significant amounts of ocean wave and tidal power energy resources. These resources are renewable and emission free for energy production. With proper system design and deployment, ocean wave and tidal power could become one of the most environmentally friendly methods for generating electricity yet developed. The Electrical Power Research Institute (EPRI) has projected that as much as 10% of the national energy demand (400 trillion watts per year) could be harnessed from US wave and tidal current energy resources.
Waves are created by winds blowing over large bodies of water; tidal changes in the sea are generated by solar and lunar gravitational forces. As the earth rotates the elliptical envelope shape of the ocean floor causes the water level to rise and fall. Ocean waves and tides contain tremendous amounts of kinetic energy which could be harnessed to turn generators for the production of electricity. Water is several hundred times denser than air, therefore, has more kinetic energy per unit speed.
It is the eastward rotation of the Earth which generates the winds which cause the waves. The Gulf Stream and the Kuroshio are two significant natural currents resulting from the Earth's rotation. The Gulf Stream starts just south of the Florida Keys and continues north for about 400 miles. The current has an average speed between 4-7 miles per hour depending on width. So long as the Earth rotates eastward, there will be winds generating waves and currents, and tidal changes will result from lunar and solar gravitational forces. This enormous power, if appropriately harnessed, is fuel cost-free, non-polluting, and self-sustaining. Furthermore, tidal currents are predictable for the indefinite future: wave patterns are predictable for days in advance. Predictability is an important characteristic for an energy source used in electrical generation which is inputted into an electric grid where the supply equals demand.
Oceanlogists calculate wave power in watts per unit wave length in meters which is a function of wave period in seconds and the square of the wave height in meters squared. The harnessing of wave energy is complex and several devices have been proposed including: point absorbers, oscillating water columns, overtopping terminators, and linear wave absorbers. Since this disclosure does not focus on wave energy, these devices are noted but not discussed in detail.
Oceanologists calculate tidal current power in watts per unit cross-sectional area in meters squared which is a function of the density of the water and the cube of the speed in meters per second. For example, the EPRI calculated the annual depth average power density of the tidal flows under the Golden Gate Bridge to be in the order of 3 Kilowatts per meter squared. The average US home uses 1.3 kW per day. The EPRI estimates that the total tidal and river potential to be in the order of 149 trillion watt hours per year or about 3.5% US national electricity demand.
The present disclosure is concerned with harnessing tidal power, which as previously discussed, is generated by lunar and solar gravitational forces as the Earth rotates eastward. The tidal currents are to be distinguished from the powerful currents occurring in the Gulf Stream which are caused by winds, uneven temperatures, and the shape of existing land masses. Tidal currents are the periodic motion of water caused by the different lunar and solar gravitational attractive forces on different parts of the eastward rotating Earth. As these gravitational forces change, tides rise and fall causing periodic horizontal movement of water, the tidal currents. The tidal current speed varies from place to place depending on the shape of the coastline being strongest in inlets, sounds, coastal waterways, and related. Since the amount of electricity generated depends on the speed and steadiness of the water driving the generating device, the tidal currents can produce electricity only between high tides and low tides.
For the above reasons, the “capacity factor” for the tidal currents is somewhat less than, for example, powerful ocean currents as in the Gulf Stream which are constant at 4-7 MPH 24 hours per day. The EPRI has estimated that with tidal units and wind units the average power is typically between 30-40% of the “rated power” which is based on a capacity factor of 24 hours per day of continuous year long operation. While the extraction rate is somewhat low, it is well worth the effort since the energy is self sustaining, non-polluting and fuel cost-free.
Tidal kinetic energy extraction is an extremely complex operation and several devices have been proposed as discussed below. Prior art most often discusses the design of these devices in terms of their physical arrangement. Water turbines, as well as wind turbines, are generally grouped into two types: 1. Vertical-axis devices in which the axis of rotation of the water/wind driven rotor is vertical to the ground and perpendicular to the energy stream, and 2. Horizontal-axis devices in which the axis of rotation of the water/wind driven rotor is horizontal with respect to the ground and parallel to the energy stream. An example of the former is the Blue Energy Ocean Turbine located in Canada. An example of the latter is the presently used wind mills in which the axis of rotation is parallel to the ground.
Generators are well known in the prior art, and similar in design and function when used in hydro-electric, wind, or ocean currents. Several models are available commercially, usable in either wind or water, provided the water unit has a water-proof housing. The kinetic energy of the water turns a rotor blade which is attached to a rotor shaft which extends into the generator. A series of step-up gears increase the rotational speed such that electricity is generated.
The rotor blades of prior art wind turbines tend to be long and narrow. The reason for this long and narrow design is that the rotors are easier to tie down and secure during violent wind storms, not that they are more efficient in capturing kinetic energy. These long and narrow rotor blades have been tried in water turbine systems, but have encountered several problems. The two major problems are they destroy fish and other marine species, and the blades are damaged by sea weeds and other submerged debris in the water.
Prior art turbines tested to date destroyed fish and other marine species to the extent that the devices have been nicknamed “chum machines”. The long, sweeping motion of the rotor blades tend to attract fish and injure them as they swim by in the sweep path of the rotor blades. In addition, these blades generate a lot of bubbles in the water. This is caused by cavitation, which is caused by difference in pressure gradients which forms vapor bubbles on the blade surfaces. While not lethal to fish, they are unsightly and may have some environmental impact.
A recent prototype test in the East River at New York demonstrated the second aforementioned problem. The weeds, debris, and other submerged material caused a failure of the prototype test when the rotor blades broke.
Prior art publications can be divided into documents disclosing designs for tidal currents and designs for stronger ocean currents; recalling tidal currents are caused by lunar and solar forces occurring in the more shallow inlets and sounds while the stronger ocean currents are in deeper water as the Gulf Stream. U.S. Pat. No. 7,378,750 issued to Herbert L. Williams, U.S. Pat. No. 6,967,413 issued to Ramez Atiya, and U.S. Pat. No. 5,051,069 issued to Peter T. Redemackerl relate to system designs for the tidal currents. The system disclosed in 750 uses a rotor design which includes hinged vanes attached to a cylindrical drum. The hinged vanes capture the water current turning the drum thereby turning the generator. The 413 disclosure involves an artificial lagoon formed by a series of support columns with a retaining wall to direct the water current toward the generator increasing its force. Patent 069 discloses a rotor with an open center with blades between an inner rim and an outer rim.
U.S. Pat. No. 7,385,302 issued to Stanley C. Johnson discloses a wind turbine having individually pivotal airfoils mounted on a rotatable frame; US 20080138206 published Jun. 6, 2008 discloses a rotor design with anti-foiling tip for shedding seaweed and other submerged debris.
Examples of patent documents disclosing tethered systems for deeper currents such as in the Gulf Stream include U.S. Pat. Nos. 7,291,936, 6,531,788, and published application 20070257492 published Nov. 8, 2007, John Robson inventor. These systems use two counter-rotating full-bladed (16 blades) rotors to turn the generators. Other examples include U.S. Pat. No. 6,726,439 issued to Amir S. Mikhall et al and U.S. Pat. No. 6,091,161 issued to James G. P. Dehlsen et al. The 439 disclosure includes a plurality of blades which can be varied in length to increase or decrease the rotor diameter. The 161 disclosure discusses variable pitch rotor blades, where the pitch is controlled by an ascend-descend protocol for controlling depth. U.S. Pat. No. 7,279,803 issued to Kenneth Randall Boslley discloses a different system design where an impeller is set in a frame and rotates around a non-moving shaft. Electromagnetic coils on the impeller and the frame generate electrical current.
The above references fail to at least teach or suggest the design of the presently disclosed and claimed invention. Other references are cited in the Information disclosure Statement.